DESCRIPTION: The long-term goal of this research is to understand the interaction of sensory cues from the otolith organs (measuring gravity and linear acceleration) and the semicircular canals (measuring rotation) and to understand the influence of the regular (tonic) and irregular (phasic) vestibular afferents on this sensory interaction. It is particularly crucial to understand these sensory interactions as we develop clinical procedures to accurately assess otolith function since all otolith responses are context dependent because of the inherent ambiguity associated with the measurement of gravity and linear acceleration (Einstein's Equivalence Principle). Specifically, the otolith organs measure gravitational cues indicating tilt as well as linear acceleration cues indicating translation. It is crucial that the nervous system be able to distinguish tilt from translation because the appropriate compensatory response to each is quite different. Other sensory cues play crucial roles in this process. For example, rotational cues measured by the canals could confirm otolith indications of tilt. Similarly, visual cues could also influence this resolution of otolith measurements and also help determine the proper gain of linear VOR responses, which depend upon target distance. Therefore, the general goal of the proposed research is to measure both tilt and translation responses during stimulation of both the canals and otoliths to try to understand how the nervous system interprets the sensory cues from the otolith organs. These sensory interactions are important because all natural movements involve combinations of cues from the otolith organs and semi-circular canals. There are five specific aims of the proposed research. 1) Investigate how the human nervous system separates otolith cues into estimates of gravity and linear acceleration during eccentric rotation. 2) Investigate the role played by irregular otolith units in eliciting the steady-state unidirectional ("bias") component of OVAR responses. 3) Investigate the role played by irregular otolith units to help the nervous system respond to tilt with respect to gravity and linear acceleration during eccentric rotation. 4) Investigate how the human nervous system processes otolith cues to induce ocular torsion. 5) Investigate the gravitational stimuli required to induce changes in the axis of eye rotation in humans. All of the proposed research relies upon a model-based approach; a model has been developed to help explain these sensory interactions and to predict eye movements. Experiments will verify or refute the model by testing the combined influences of the otolith organs and semicircular canals on reflexive eye movements.